This invention relates to acoustic devices for which acoustic performance relies on bending wave action and related beneficial areal distribution of resonant modes of surface vibration.
For first teaching regarding such resonant mode acoustic devices, particularly as panel-form members, reference is directed to International Patent Application WO97/09842; or its U.S. equivalent, application Ser. No. 08/707,012, filed Sep. 3, 1996 and various later patent applications by New Transducers Limited before this patent application make useful additions and developments, including as to viable variations of transducer location(s) and/or panel distribution(s) of bending structure/geometry and/or stiffness and/or mass.
This invention arises particularly in relation to loudspeakers using such panel-form members as acoustic radiators.
A typical panel-form loudspeaker comprises a distributed mode acoustic radiation member having a moving coil drive unit to impart bending waves to the panel by push-pull action applied to the surface of the panel. The typically circular section voice coil of the drive unit exerts force by its end in circumferential, contact with the panel member. A typically circular zone of the panel member effectively within the voice coil sectional area can both resist desired formation of bending waves in the panel member, and itself vibrate to produce acoustic output components at high frequencies by way of drum-like action (xe2x80x9cdrummingxe2x80x9d).
It is an object of the invention to aid useful coupling between drive units and panel members of loudspeakers either to decrease resistance to bending wave formation or to reduce high frequency drumming, ideally both further hopefully usefully to increase and/or smooth energy input and/or frequency response/output.
According to one aspect of the invention a loudspeaker comprises a panel-form member as resonant acoustic radiator and a driver coupled to the member to apply bending waves thereto, and is characterised in that the panel member is altered or different locally of the driver as coupled to the panel member in a manner involving mass and/or bending capability.
The alteration or difference in mass and/or bending capability may be within the confines or area of the coupling of the driver to the panel member, and may be concentric with the driver.
Localised reduction of mass and/or bending resistance may be achieved by removal or absence of portion(s) of the panel member. At least one hole made in or through the panel member may be of substantially constant or tapered cross-section; or the hole may be mis-matched to the coupled end of the driver, as the voice coil of magnet-and-moving coil type said driver, to facilitate spaced connections of said coupling, such as in our co-pending UK patent application GB 19709438.
Such a hole through the panel member within the area of the driver, typically voice coil coupling removes panel material which could otherwise resonate in drum-like manner. Such a hole also militates against what could otherwise effectively be stiffening by the driver coupling. Efficiency of power transfer into the panel member may be usefully increased. Reduction of mass of the panel member near the voice coil, and the presence of an xe2x80x9cedgexe2x80x9d within the excitation area can assist bending wave formation and acoustic radiation, with effective reduction of unwanted high frequency content from drumming effects. Diffraction resulting from such a hole/edge can be reduced by various means including extending the drive unit pole piece into the hole, or adding other material to the pole piece, say to make it level with the panel surface.
Such a hole can allow the possibility of fixing the voice coil former right through the panel member, skin-to-skin, to increase the strength of fixing bond and to allow higher powers to be applied to the panel member without damaging the structure.
The hole in the panel member if non-parallel sided, typically conical tapered from one panel side, can, if of less than full thickness of the panel member, be nondamaging to the cosmetic appearance of the other side, say front, of the panel member.
Suitable apertures or holes in the panel member, particularly through the area of driver coupling thereto, may range up to cross-sectional size of the vibration-inducing driver component, usually voice coil. Different hole sizes produce different upward shifts of unwanted high frequencies arising from drumming, thus enable extension of acoustic working frequency range to desired extent of reduction of intrusion/content from drum-like vibration.
The alteration or difference in mass and/or bending capability of the panel member may be by way of affixing an additional mass that may be on either or both sides of the panel member within the area of the driver, typically voice coil, coupling. Such additional mass may primarily mass-load the panel member; or primarily provide additional damping to the panel member; or have the combined effect of mass-loading and damping the panel member. There is a greater tendency to reduce wanted acoustic output frequencies below those that otherwise would be attainable without modification (due to drum-like vibration) by adding a mainly mass-effective mass than by adding mainly damping-effective material or using holes.
Suitable affixed mainly mass-effective means, which may be small, typically fraction of a gram, will serve as a load that reduces efficiency of bending involved in drum-like vibration, thus at least reducing amplitude of unwanted high frequency acoustic components of drumming. Size of the affixed mass should not be more than achieves acceptable compromise between desired reduction of unwanted high frequencies and inevitable accompanying reduction of adjacent wanted frequencies.
There is another advantageous use for typically similarly small added/affixed masses in the control of otherwise at least potentially overly excited acoustic output frequencies, namely at feasible but unused in-board preferential driver coupling locations, with the useful effect for the acoustic output of the panel member that it is beneficially quieted and smoother. This is, of course, applicable both with and without other control(s) hereof in relation to drumming, and thus of independent inventive merit.
Suitable affixed mainly damping-effective material, usually of small to smallest possible mass, but say up to what might otherwise now feasibly further be effective as affixed mass, will serve by stretching and contracting to absorb energy in and of bending for drum-like vibration. The size/bulk of the affixed damping material need not be more than enough to dissipate desired/useful amount of energy, thus reduction of amplitude of high drumming frequencies, feasibly with such small mass as to have little or virtually no effect on adjacent wanted acoustic output frequencies. Typical damping material will be of light-weight elastomeric nature.
There are further advantageous uses for affixing of elastomeric material to the panel member. One is where damping material basically for effect on drumming as above is such that, or is associated with a driver configured such that, the material is or can usefully be sandwiched between a driver part (typically magnet pole-piece), and surface of the panel member within the driver coupling (typically voice coil), and with or without some operatively useful degree of effectively pre-compression. The other is where such elastomeric material in sandwiched relation with driver part and panel member surface serves in suspension of that driver part, typically requiring damping/spring compliance hitherto provided by spring means, whether as wholly or partially replacing or augmenting such known provision.